Dissolved gases monitoring to disclose regional hydrogeochemical interactions processes involved in mineral water genesis in non-active zone: the case of Corsica Island
Résumé
In complex geological context, not seismically active, deep
water-rock interactions associated with transit time and
geological disparities can generate singular mineralisation
associated to significant gaseous content. Widely used as tracers
in seismically – and tectonically– active regions, the gaseous
emission monitoring is underused to discriminate regional from
local groundwater flow patterns. In this study the dissolved gas
monitoring in groundwater has proven to be an innovative tool
for reconstructing interactions involved in regional flows, and
responsible for the genesis of highly diversified mineral water-
types.
The oriental plain of Corsica (France) has been investigated as
an area with a wide variety of mineral waters (22 springs)
emerging at the interface of magmatic, metamorphic and
sedimentary rocks. Dissolved reactive (N2, CO2, CH4, H2S, H2,
O2) and noble gases (Ne, Ar, He) on 9 springs have been
quarterly sampled, and analysed by gas chromatography (μGC).
The first results highlight 3 very contrasted gas abundances:
N2-rich thermal waters (54 °C), poorly mineralised,
with noble gas occurrence as cortege gases, highlighting
the influence of deep flow with a long groundwater
residence time.
CO2-rich cold waters (<20 °C), low to highly
mineralised, with N2 as cortege gase, highlighting the
occurrence of deep flow interacting during upflow
through metacarbonates.
CH4-rich cold waters (<20 °C), highly mineralised,
with H2S and CO2 as cortege gases, showing biotic
anaerobic activity involvement in the gases composition
of waters.
Then, based on the observed abundance of noble gases,
theoretical recharge conditions were computed to defined
recharge temperature, air- and He-excess. Computation results
have stressed out the common origin of these three gas,
depending on flow paths, reservoir conditions, biotic and abiotic
interaction involvement. The circulation within magmatic
reservoir is responsible for the deep N2-rich flow, which shows
during his up-flow abiotic interactions with metamorphised carbonates rocks, increasing the CO2 content in water. Then
under anoxic geological confinement in deep sedimentary layers,
the CO2 is reduced into CH4 and N2 into NH4. In the shallowest
sedimentary layers, the formed CH4 is degraded, due to the
occurrence of rich-organic matter lithology, by biotic activity
into H2S.